Abstract: n-Alkenes can be converted catalytically into isoalkenes, when a mixture containing at least one n-alkene and hydrogen is brought into contact at a temperature of 400.degree. to 600.degree. C. and a residence time of 0.2 to 20 seconds with an Al.sub.2 O.sub.3 or SiO.sub.2 catalyst which is specifically activated and is promoted with halogen compounds of elements of the third to fifth main groups and second to eighth sub-groups of the periodic system and/or hydrohalic acids. The specific activation comprises calcining, before the promotion process, an aluminum hydroxide or aluminum oxide hydrate or a silica gel having a content of volatile ammonium salts to give Al.sub.2 O.sub.3 or SiO.sub.2, with expulsion of these volatile ammonium salts. For the conversion of the n-alkenes, the catalyst can be employed in a fixed or a mobilized catalyst bed.